This invention relates generally to actuators and sensors used in precision spacecraft structures and, more particularly, to techniques for minimizing the effects of temperature changes on such structures. It is well known that certain materials exhibit a piezoelectric or electrostrictive effect and may be used as actuators, wherein an applied electric field causes mechanical strain or deformation along a selected axis of the material. An inverse effect allows strain to be sensed as a generated electrical signal. Devices of this type are especially useful in space applications, such as for vibration suppression, vibration isolation, active damping, health monitoring and shape control. Actuators or sensors can be conveniently incorporated into "host" structural members as implants or as patches applied externally.
A well recognized difficulty in any precision application of ceramic piezoelectric or electrostrictive sensors and actuators is that the actuator/sensor material may have a non-zero coefficient of thermal expansion (CTE) that does not match the CTE of the host structural element. This mismatch of CTEs can cause unwanted bending or other strain in the host structure, and can cause errors in sensed strains in the structure. U.S. Pat. No. 5,305,507 issued to George R. Dvorsky et al., entitled "Method for Encapsulating a Ceramic Device for Embedding in Composite Structures," discloses a technique for encapsulating an actuator/sensor using a piezoelectric ceramic material, such as lead zirconate titanate (PZT) or lead magnesium niobate (PMN) (ceramic). Basically, the technique disclosed in the Dvorsky et al. patent is to design the encapsulating material to provide strain relief to offset the brittleness of the ceramic actuator and to provide electrical isolation for the ceramic actuator and leads. The effect of encapsulation, however, resulted in an actuator/sensor that has a net positive CTE. For applications requiring high precision, thermal distortion of structures must be limited or highly controlled. Temperature changes produce expansion or contraction when non-zero CTEs are present, and mismatching of CTEs can cause unwanted structural bending.
The present invention focuses on minimizing the net encapsulant CTE or matching the net encapsulant CTE to that of the host structure. An additional desire is to simultaneously maximize the actuation strength using a suitable figure of merit. The net result is a thermally stable sensor/actuator package.
In general, negative CTE materials are accompanied by high modulii. A practitioner skilled in the art recognizes that the steps taken to minimize the CTE reduce the actuation strength. That is, alignment of the high modulus fibers in the direction of desired low CTE results in increased stiffness. Maximization of actuation strength competes against this objective by requiring decreased stiffness. An important objective of this invention is to optimize both characteristics: CTE and actuation strength.